This application relates to improved processes for the chemical production of chlorine from gaseous HCl. The gaseous HCl can be part of a complex mixture.
In recent years increasing amounts of HCl are being obtained as a by-product of several manufacturing processes such as the industrial production of chlorinated hydrocarbons. At the same time industrial demand for gaseous chlorine has also greatly increased. As a result there is a great need for more efficient chemical processes for producing chlorine from hydrogen chloride, especially processes capable of large-scale industrial application.
In 1868, Deacon developed a process by which chlorine is produced by direct oxidation of gaseous HCl with O.sub.2 in the presence of a CuCl.sub.2 catalyst. This process is described by the overall chemical equation EQU HCl(g)+1/4O.sub.2 (g).fwdarw.1/2H.sub.2 O(g)+1/2Cl.sub.2 (g).(1)
Reaction (1) in the presence of a CuCl.sub.2 catalyst is a fast overall exothermic process, which is expected to reach equilibrium under normal industrial operating conditions of 700.degree. K. to 750.degree. K.
A number of engineering problems are associated with the Deacon process. The temperatures of the process reduce the equilibrium constant for the conversion, resulting in incomplete conversion of the HCl and thereby reducing yield. This is especially a problem when the Deacon process is carried out at a single temperature in a single vessel. Furthermore, at elevated temperatures above 675.degree. K., the catalyst's activity rapidly decreases, mainly because of volatilization of the CuCl.
Since the early 1900's various efforts have been made to improve the Deacon process. Several modifications of the catalyst's composition have been suggested, such as addition of less volatile rare earth metals in the form of chlorides or oxides, and addition of various copper salts, which are promoted by chlorides or oxides of a number of metals such as V, Be, Mg, Bi, and Sb. Several researchers have proposed the addition of NaCl and KCl, which form double salts with the CuCl. These double salts are less volatile than the CuCl itself. Cr.sub.2 O.sub.3 and V.sub.2 O.sub.5 have also been shown to be efficient catalysts for the process. However, few, if any, of these modifications have been shown to improve efficiency of the process under actual industrial operating conditions.
Particular modifications of the Deacon process are described in U.S. Pat. No. 2,206,399 to Grosvenor et al., U.S. Pat. No. 2,577,808 to Pye et al., and J. Th. Quant, J. van Dam, W. F. Engel, and F. Wattimena, "The Shell Chlorine Process," The Chemical Engineer, 224-232 (1963).
U.S. Pat. No. 2,206,399 to Grosvenor et al. discloses the chlorination and oxidation of a variety of chlorine-carrying multivalent metals, including chromium, cobalt, copper, manganese, nickel, magnesium, and iron. The metal is preferably iron.
U.S. Pat. No. 2,577,808 to Pye et al. discloses the use of iron as a chlorine carrier in a fluidized bed reaction where a granular contact mass including ferric oxide falls by gravity through a heating or cooling zone, and then through a chloridizing zone which has a temperature of 300.degree. C. at the top and 500.degree. C. at the bottom. Thereafter, the chlorine carrier, now in the form of ferric chloride, falls into an oxidizing zone which has a temperature of 500.degree. C. at the top and 550.degree. C. at the bottom to oxidize the particles to ferric oxide. These particles are then returned to the top, cooled, and recycled.
The Chemical Engineer article by J. Th. Quant et al. describes a variation of the Deacon process using a Cu catalyst adsorbed on a porous carrier containing alkali metal chlorides and/or lanthanide chlorides, the so-called "Shell catalyst." The reaction is optimally carried out in a fluidized bed.
A number of other processes have been proposed for the recovery of Cl.sub.2 from waste HCl. These processes include:
(1) The Kel-Chlor Process. This process involves the reaction of HCl with nitrosylsulfuric acid (HNSO.sub.5) contained in a stream of H.sub.2 SO.sub.4 to produce nitrosyl chloride (NOCl) with eventual production of Cl.sub.2 by oxidation of NOCl.
(2) Direct Electrolysis of Hydrochloric Acid.
(3) Direct Oxidation with an Inorganic Oxidizing Agent. Such oxidizing agents include nitrogen dioxide, sulfur trioxide, or a nitric/sulfuric acid mixture. The reaction is carried out in the liquid phase.
(4) Weldon Process. This process is based on the oxidation of aqueous hydrochloric acid with manganese dioxide, with subsequent reconversion of manganous chloride by air blowing in the presence of lime.
None of these processes can be characterized as completely successful. Direct electrolysis of HCl is only exploitable where power costs are low and the recovered co-product, hydrogen, can be made to bear an appropriate share of the manufacturing costs. In the present industrial environment of high and unpredictably fluctuating energy costs, such a process has little use. The processes involving direct oxidation with an inorganic oxidizing agent are very corrosive and give relatively low yields of chlorine. The various two-stage processes, including the Weldon process and variations of the Deacon process, attain far lower conversions under normal industrial operating conditions than are claimed to occur theoretically. Also, catalytic activity decline and loss due to catalyst volatilization still remain severe problems and major components of the final product cost. The Kel-Chlor process is very costly in plant design, safety features, and energy requirements.
Accordingly, there is a need for an efficient process for the preparation of chlorine from HCl that gives a nearly quantitative conversion of HCl to chlorine, operates under conditions in which the catalyst does not volatilize and in which the activity of the catalyst remains stable, and operates at relatively moderate temperatures to prevent corrosion and minimize the extrinsic energy input required.